1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include a network of base stations, base station routers, and/or other wireless access points that are used to provide wireless connectivity to mobile units in geographic areas (or cells) associated with the network. Information may be communicated between the network and the mobile units over an air interface using wireless communication links that typically include multiple channels. The channels include forward link (or downlink) channels that carry signals from the base stations to the mobile units and reverse link (or uplink) channels that carry signals from the mobile units to the base station. The channels may be defined using a time slots, frequencies, scrambling codes or sequences, or any combination thereof. For example, the channels in a Code Division Multiple Access (CDMA) system are defined by modulating signals transmitted on the channels using orthogonal codes or sequences. For another example, the channels in an Orthogonal Frequency Division Multiplexing (OFDM) system are defined using a set of orthogonal frequencies known as tones or subcarriers.
Next (4th) generation wireless systems such as 802.16e WiMAX, UMTS Long Term Evolution (LTE) and cdma2000 EV-DO Revision C Ultra Mobile Broadband (UMB) are based on Orthogonal Frequency Division Multiple Access. In OFDMA, the transmitted signal consists of narrowband tones that are nearly orthogonal to each other in the frequency domain. A group of tones transmitted over the duration of one time slot (or frame) constitutes the smallest scheduling resource unit, also known as a “tile,” a resource block (RB), or a base node (BN). Different tones belonging to a tile may be scattered across the entire carrier frequency band used by the OFDMA system so that each tile transmission experiences diversified channels and interference on each sub-carrier. Alternatively, a tile can be formed of a contiguous set of tones so that the channel and interference experienced by the tile are more localized. Hybrid Automatic Repeat reQuest (HARQ) is employed to increase the capacity of the OFDMA system. To this end, the encoder packet transmission includes multiple HARQ interlaces repeating every certain number of frames and having a fixed maximum allowed number of sub-packet retransmissions. In systems such as UMB that employ synchronous HARQ, a scheduler allocates tile-interlace resources for the duration of each encoder packet transmission. This approach is conventionally referred to as “packet-based” allocation.
OFDMA is a fully scheduled medium access control scheme on both the uplink and downlink channels and therefore requires explicit signaling to assign resources to user data transmissions. Since the resources allocated to each packet can differ, packet-based resource allocation requires signaling overhead for each packet. The signaling overhead needed to provide this assignment can consume a significant amount of bandwidth and/or power, especially when the system supports a large number of relatively low rate applications such as Voice over Internet Protocol (VoIP). One alternative to the packet-based resource allocation scheme is persistent resource assignment. In a persistent resource allocation scheme, channel resources (such as frequency tones and/or HARQ interlaces) are assigned for the duration of a voice talk spurt instead of being assigned for each individual VoIP packet transmission. Persistent resource allocation relies upon the well-known property that conversational voice traffic is generated in talk spurt followed by periods of silence that correspond to a listening mode. The average duration of a talk spurt is around 1-2 seconds and therefore consists of many VoIP packets, which are generated every 20 ms in the case of a CDMA2000 EVRC vocoder. Since signaling overhead is only required at the beginning of the talk spurt when the persistent resource allocation is performed, the overhead for talk spurt assignment is significantly lower than the overhead required for packet-based resource allocation.
However, persistent resource allocation has a number of drawbacks. For example, allocation of resources to a talk spurt may be delayed because traffic resources that could be assigned persistently to the new talk spurt on a sustained basis may not be available. For another example, the radiofrequency conditions associated with the user requesting the persistent resource allocation may require the use of multiple contiguous tiles (also known as OFDMA base nodes or resource blocks) across different interlaces. However, only non-contiguous resources may be available at the time of talk spurt arrival. The decision to assign persistent resources may also require more time because the scheduler may need to consider factors that are not as important in packet-based resource allocation. Exemplary factors that may be considered during persistent resource allocation include resource balancing, overload control, and the impact of persistent resource assignment on the quality of service of existing users. Furthermore, system outage and dynamic optimization of available traffic resources should be performed because the scheduling decision for persistent resource assignment is made for substantial periods of time instead of on a packet-by-packet basis. Non-persistent assignments typically ignore or greatly simplify these considerations.
The delay in assigning persistent resources may degrade the quality of service perceived by users. For example, if the start of the talk spurt is delayed beyond a certain bound, the users participating in the conversation will not have a natural experience when they try to interrupt each other. For example, the delay may cause the talking user to believe that the listening user has not heard what the talking user said, which may cause the talking user to repeat what they have said. The listening user may also respond later than expected, causing the users to believe that they are talking at the same time. Controlling the latency of the voice packets at the start of the new talk spurt is more important than the latency of the other packets in the middle of the talk spurt. For example, when the start of a new talk spurt is delayed, the jitter and/or packet inter-arrival requirements for subsequent packets may make it difficult to correct for the initial delay before the end of the talk spurt.